1. Field of the Invention
The present invention relates to a quartz crystal unit for high frequency such as 300 MHz or more, and particularly to a crystal unit that has a reduced change in oscillation frequency when it is mounted on a wiring board by reflow soldering or the like.
2. Description of the Background Art
Crystal units with a quartz crystal blank contained in a casing are integrated into oscillators or filters in various electronic devices as a reference source of frequency or time. Crystal units are now designed to have higher oscillation frequencies as they are also integrated into optical communication systems. For achieving such higher oscillation frequencies, a crystal unit is developed in which an oscillation region of a crystal blank has a recess to reduce the thickness of the crystal blank at the recess thereby increasing the oscillation frequency, and the relatively thicker portion around the recess supports the oscillation region to maintain the mechanical strength.
FIG. 1 is an exploded perspective view of such a conventional crystal unit. The crystal unit comprises casing body 1 composed of ceramics or the like and having a recess, and crystal blank 2 contained in casing body 1. Casing body 1 substantially has a rectangular parallelepiped shape, and crystal blank 2 substantially has a rectangular shape. A step is provided in one side of the recess of casing body 1. A pair of connection terminals 3 for electrical connection to the crystal blank are provided at both ends of top of the step. A pair of mounting terminals (not shown) used for surface mounting of the crystal unit on a wiring board are provided on the outer surface of casing body 1. The mounting terminals are connected to connection terminals 3 through via holes provided in casing body 1.
The configuration of crystal blank 2 is shown in FIGS. 2A and 2B in detail. Crystal blank 2 typically comprises an AT-cut quartz crystal blank. The AT-cut crystal blank has a resonance frequency that depends on its thickness, and a smaller thickness thereof produces a higher oscillation frequency. Accordingly, in order to achieve a higher oscillation frequency, hole 4 is provided in a center portion of one main surface of crystal blank 2 so that the crystal blank has at the bottom of hole 4 a smaller thickness than the thickness at the peripheral portion, thereby defining the portion of reduced thickness as an oscillation region. Excitation electrodes 5 are provided on opposing main surfaces of crystal blank 2 in the oscillation region. Extending electrodes 6 extend from the respective excitation electrodes 5 towards respective opposing sides of one shorter edge of crystal blank 2. The pair of extending electrodes 6 are to be associated with the pair of connection terminals 3 provided on the step of casing body 1. A tip end of extending electrode 6 provided on the shown, upper surface of crystal blank 2 turns at the afore-mentioned shorter edge onto the shown, lower surface of crystal blank 2.
Excitation electrode 5 and extending electrode 6 have a two-layer structure consisting of an under-layer electrode (i.e., adhesion film) deposited directly on crystal blank 2, and a conduction electrode layer formed on the under layer electrode. The first, under layer electrode is formed of a material such as chromium, a nickel-chromium alloy, or a nickel-tungsten alloy, and the second, conduction electrode layer is formed of, for example, gold. The under-layer electrode and conduction electrode layer are deposited by a method such as vacuum evaporation, or sputtering.
The opposing sides of one shorter edge of crystal blank 2, which have been described above, are fixed to the pair of connection terminals 3 by adhesive members 7 such as conductive adhesive, so that crystal blank 2 is retained horizontally in the recess of casing body 1, and connection terminals 3 and extending electrodes 6 are electrically connected, thereby electrically connecting the mounting terminals provided on the outer surface of casing body 1 and excitation electrodes 5 of crystal blank 2.
After fixing crystal blank 2 to the step of the recess as described above, the open side of recess is sealed by cover 10 so that crystal blank 2 is hermetically sealed in the casing. When integrating the crystal unit thus formed into an actual circuit, the crystal unit is mounted on a wiring board, and fixed to the wiring board by, for example, reflow soldering.
However the above-described crystal unit has a problem that change in oscillation frequency occurs through mounting process, when it is transferred in a furnace for reflow soldering in the mounting process onto a wiring board. In particular, this change in oscillation frequency is more prominent for crystal units having higher nominal oscillation frequencies.